The present invention relates generally to an assembly and adjusting method of an optical system, and more particularly to an assembly and adjusting method of a projection composed of a multilayer mirror used for an EUV exposure apparatus.
Reduction projection exposures using ultraviolet have been conventionally employed to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit in lithography technology. However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device. Therefore, an exposure apparatus using extreme ultraviolet (“EUV”) light with a wavelength of approximately 13.5 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer very fine circuit patterns of 50 nm or less.
The EUV exposure apparatus uses a reflection-type optical element such as a mirror for an optical system, and a multilayer film that alternately forms or layers two kinds of materials having different optical constants is formed on a surface of the reflection-type optical element. The multilayer film is formed by alternately forming or layering molybdenum (Mo) layer and silicon (Si) layer on a precisely polished glass plate. The thickness of the layer is decided according to the wavelength of the exposure light etc., and for example, a Mo layer is about 3 nm thickness, and a Si layer is about 4 nm thickness. A sum of thickness of two kinds of materials is generally called a coating cycle, which is 7 nm in the above example.
The multilayer mirror reflects EUV light with a specific wavelength when receiving EUV light. Efficiently reflected EUV light is one within a narrow bandwidth around λ that satisfies an interference condition where λ is a wavelength of the reflected EUV light, θ is an incident angle and d is a coating cycle and the bandwidth is about 0.6 to 1 nm. The interference condition is approximately expressible by Bragg's equation (Equation 1), but it shifts minutely from a value obtained from this equation strictly due to refraction in the material etc.2×d×cosθ=λ  (1)
On the other hand, the multilayer mirror in the projection optical system requires very high precision for its surface shape. For example, a permissible figure error σ (rms value) is given in Marechal's equation (Equation 2) below where n is the number of multilayer mirrors in the projection optical system, and λ is a wavelength of the reflected EUV light.
                    σ        =                  λ                      28            ×                          n                                                          (        2        )            
For example, six multilayer mirrors in the projection optical system that uses the exposure light with a wavelength of 13 nm is permitted to have a figure error σ of 0.19 nm. The wave front aberration amount is about 0.4 nm (rms value) for resolution of 30 nm pattern transfer, which is permitted for the whole projection optical system.
The projection optical system fabrication method includes a forming process of the multilayer mirror, a shape measuring process, an assembly process to a lens barrel, and an adjusting process of the wave front aberration.
The multilayer mirror forming process polishes the substrate while repeating the shape measurement with the interferometer that uses visible light, and forms a predetermined shape substrate. Next, the multilayer film is formed on the substrate surface. When actually functioning as the optical system, a best thickness distribution is formed in consideration of the angle and the wavelength of the light irradiated to each position of the multilayer film on the mirror surface.
The shape measuring process measures the surface shape of the multilayer mirror that completes the forming the multilayer film by the interferometer that uses visible light, and judges whether the surface shape of the multilayer film satisfies the predetermined shape (in other words, above figure error a). The multilayer mirror judged not to have the predetermined surface shape exfoliates the multilayer because the forming the multilayer film is a failure, and re-forms the multilayer film.
The assembly process to a lens barrel sets the multilayer mirror judged for the shape measuring process to have the predetermined surface shape in the lens barrel, and adjusts an interval and inclination between mirrors. This completes projection optical system.
The adjusting process of the wave front aberration adjusts the wave front aberration of the projection optical system. If a phase change of light by the reflection is a constant value, the wave front of the reflected light reflected from the mirror can be obtained from a wave front of the incident light to the projection optical system and the mirror shape. However, actually, the phase change of the reflected light reflected from the multilayer mirror is different depending on the wavelength of light, the incidence angle, and the film structure. Therefore, even if a geometrical surface shape is measured by visible light, the reflected light surface when the EUV light is irradiated can not be accurately obtained. Then, a method of directly measuring the reflection light surface of the multilayer mirror or the projection optical system by using the EUV light executed limitedly. For example, a point diffraction interferometer (PDI) that generates a spherical wave by a pinhole is known as a means to directly measure the reflected light surface of the multilayer mirror by using EUV light. U.S. Patent Application Publication No. 2002/044287 discloses the PDI.
A method that obtains the layer structure of the X-ray multilayer mirror and the information of interface roughness from the X-ray standing wave spectrum form is known as other prior art. For example, Japanese Patent Application Publication No. 2000-55841 and Japanese Patent Application Publication No. 2002-243669 disclose these techniques.
The model calculation concerning the relationship between the reflectivity of the multilayer film and the phase of the reflected light has been disclosed (see, for example, J. H. Underwood and T. W. Barbee, “Layered Synthetic Microstructures as Bragg Diffractors for X-Rays and Extreme Ultraviolet: Theory and Predicted Performance”, Applied Optics 20, 3027 (1981)). Moreover, the photoelectric effect of the multilayer film has been disclosed (see, for example, Michael E. Malinowski, Chip Steinhaus, W. Miles Clift, Leonard E. Klebanoff, Stanley Mrowka, Regina Soufli, “Controlling contamination in Mo/Si multilayer mirrors by Si surface capping modifications”, Proc. SPIE Vol. 4688, Page 442-453, July 2002).
However, in PDI method, because a size of the pinhole used to generate an accurate spherical wave is very minute (for example, plural tens nm), and the fabrication of the apparatus is difficult. It is necessary to introduce the EUV light with enough intensity for the minute pinhole. Moreover, PDI method measures the EUV light reflected by all mirrors that compose the optical system. For example, a light intensity of the light that exits from the projection optical system decreases to only 8% of a light intensity of the incident light, where the number of the mirror that composes the projection optical system is six, and the reflectivity is 65%. Therefore, it is necessary to use a high-intensity light source to measure the entire projection optical system, and the measurement system becomes larger and more expensive.
Although the method disclosed in Japanese Patent Application Publication No. 2002-243669 can simply measure the multilayer mirror shape, the wave front of the reflected light can not be obtained if the phase is not considered. Therefore, the wave front of the reflected light can not be correctly obtained, enough in the adjustment of the wave front aberration.